Gandalf is concerned about Gollum for two reasons:
- Because without Gandalf being so concerned, the cause would have failed.
- Because Gandalf himself is inherently moral, as an angelic representation who has not succumbed to temptation (a la Sauron, Saruman).
Tolkien explicitly addresses the former point in his letters:
In this case the cause (not the 'hero') was triumphant, because by the
exercise of pity, mercy, and forgiveness of injury, a situation was
produced in which all was redressed and disaster averted. Gandalf
certainly foresaw this. See Vol. I p. 68-9. Of course, he did not mean
to say that one must be merciful, for it may prove useful later – it
would not then be mercy or pity, which are only truly present when
contrary to prudence. Not ours to plan! (Letter 192)
To paraphrase the relevent paragraph as Gandalf himself states, no one can see the future but it's his own gut feeling that Gollum would still be relevent, so killing him or at a minimum treating him reasonably well could be a detriment to their cause:
even the very wise cannot see all ends. ... he is bound up with the
fate of the Ring. My heart tells me that he has some part to play yet,
for good or ill, before the end; and when that comes, the pity of
Bilbo may rule the fate of many – yours not least. (Fellowship of the Ring)
It is shown in the LotR that pity and mercy are key to the success of the quest to destroy the Ring:
‘Pity? It was Pity that stayed his hand. Pity, and Mercy: not to
strike without need. And he has been well rewarded, Frodo. Be sure
that he took so little hurt from the evil, and escaped in the end,
because he began his ownership of the Ring so. With Pity.’ ... Then do
not be too eager to deal out death in judgement. For even the very
wise cannot see all ends. I have not much hope that Gollum can be
cured before he dies, but there is a chance of it. (Fellowship of the Ring)
Further this saving of the cause by pity and mercy, Bilbo's (not to kill Gollum under the Misty Mountains), Frodo's and Sam's (not to kill Gollum when he was captured in the Emyn Muil) and Gandalf's (the well-treating of Gollum once captured), was explicitly mentioned a number of times in the letters:
at this point the 'salvation' of the world and Frodo's own 'salvation'
is achieved by his previous pity and forgiveness of injury. At any
point any prudent person would have told Frodo that Gollum would
certainly betray him, and could rob him in the end. To 'pity' him, to
forbear to kill him, was a piece of folly, or a mystical belief in the
ultimate value-in-itself of pity and generosity even if disastrous in
the world of time. He did rob him and injure him in the end – but by a
'grace', that last betrayal was at a precise juncture when the final
evil deed was the most beneficial thing any one cd. have done for
Frodo! By a situation created by his 'forgiveness', he was saved
himself, and relieved of his burden. (Letter 181)
and
He (and the Cause) were saved – by Mercy : by the supreme value and
efficacy of Pity and forgiveness of injury. (Letter 191)
I also believe, as per the latter point, that this concern gives us some insight into Gandalf's character himself.
We know that pity, mercy and compassion are some of the highest virtues as held by Tolkien, as imbued by his Christian beliefs:
They tend to forget that strange element in the World that we call
Pity or Mercy, which is also an absolute requirement in moral
judgement (since it is present in the Divine nature). In its highest
exercise it belongs to God. (Letter 246)
Gandalf is therefore presented as a very moral being (albeit not omniscient), indeed an angel:
I wd. venture to say that he was an incarnate 'angel' (Letter 156)
I think the combination of these two points gives the best explanation of why Tolkien had Gandalf showing Gollum the pity and mercy he did not deserve.
